Transistor switching circuit instability

Attached is a circuit I have been using for a few months to allow a WRT54G wireless router to remotely control a 220v pump motor for irrigation. (Not Arduino related but...)

Lately I have noticed that it will sometimes spontaneously go from "off" to an "oscillating" state, that is, the contactor turns on and off about 5-10 times per second.

Is there something I'm overlooking in my calculations or design?

I think the gpio from the router will source/sink 4ma.

The SSR is an OAC5 http://www.opto22.com/documents/0449_Std_Dig_AC_Outputs_data_sheet.pdf. It wants input 2.5-8v and 12ma.

It may be a week or two until I can actually go to the location to try and diagnose. Even then I'm not sure how to diagnose since it is intermittent. But there is a second pump installation there which uses the same circuit and is not having the problem. Of course it could be a bad component or a loose connection...

Thanks! John

Pump_interface.png|1160x591

Impossible to say from this limited information what the problem is, but why have the contactor? Many, if not most SSRs handle 220VAC. I've been using one with a 220VAC deep well pump for years with no problems at all.

jremington: Impossible to say from this limited information what the problem is, but why have the contactor? Many, if not most SSRs handle 220VAC. I've been using one with a 220VAC deep well pump for years with no problems at all.

Thanks for your reply. I used these SSR's because I had them from an old project.

And, I'm not sure about sizing and heat sinking that would be needed for my 2hp pump? From a bit of googling it looks like I would need an SSR rated for >= 75 amps. Combined with my <=4ma input requirement, suggests a $80-$100 SSR (Mouser prices).

Thanks, John

You could make the 1K into 2 X 4.7k and add a capacitor from the common junction to ground. Try .1 to 1uf

LarryD: You could make the 1K into 2 X 4.7k and add a capacitor from the common junction to ground. Try .1 to 1uf

I think I understand the idea of the cap across the input of the SSR. But what is the purpose of changing the base resistor from 1k to "2 x 4.7k"? Do you mean in series (9.4k) or parallel (2.3k)?

Thanks! John

Sorry, my mistake. 2 X 470 ohm resistors = 940 ohms to replace the 1K

But there is a second pump installation there which uses the same circuit and is not having the problem. Of course it could be a bad component or a loose connection...

There is nothing obviously wrong with the circuit, so I would agree with the last sentence. Only on site trouble shooting will tell for sure.

Try an RC combination over the contactor contacts 0.1 uF and 100 ohms are a good start.

Pelle

0.1 uF and 100 ohms are a good start

Hopefully in series, and that would need to be at least a 330VAC bipolar capacitor. Look up "RC snubbers" for more info. I personally doubt that will fix the problem, though.

You need to find out where the oscillating is comming from. It is most likely that it is the other side of the opto isolator, not the bit you have drawn. This can. E caused by faulty components or more likely interference form other controllers. A mains filter on these circuits and the one giving you trouble could help.

Almost certainly mains transients from the output of the SSR are getting back to the router. Add a mains filter to the supply for the router perhaps? Make sure none of the low voltage wiring from router to SSR runs anywhere near the main wiring. Do you have a snubber across the SSR appropriate for the pump motor?

Thank you all for the ideas, I will look into the wiring layout, snubber, and "mains filter".

Cheers, John

Grumpy_Mike: You need to find out where the oscillating is comming from. It is most likely that it is the other side of the opto isolator, not the bit you have drawn. This can. E caused by faulty components or more likely interference form other controllers. A mains filter on these circuits and the one giving you trouble could help.

By "mains filter" do you mean something like a consumer grade "surge suppressor"?

Thanks, John

Something like this:- http://www.bulgin.co.uk/Products/Mains_Filters/Mains_Filters.html

I was able to get to it yesterday with my dso nano (thanks kculm!) and pretty easily found a connection fault which was causing the problem (shown attached, blue line). Thanks all for your input! Cheers, John

pump_interface_fault.PNG|732x530